Conventional infrared detecting elements generally include quantum detectors and thermal detectors. Quantum detectors can provide excellent responsiveness and sensitivity in measuring temperature, however, they must be cooled to a temperature lower than the temperature of liquid nitrogen for use and thereby have a major disadvantage in miniaturizing an infrared detector system in addition to being relatively expensive. Thermal detectors offer advantages in that they permit a miniaturization to provide low weight and relatively low cost and do not require an active cooling system. Thermal detectors, however, provide limitations in their sensitivity, and substantial efforts have been made in this field to address these problems. One approach with a thermal IR detector designed to absorb IR radiation is to provide a free-standing membrane that is suspended by supports directly from a heat sink. The absorbed IR radiation causes a temperature rise in the membrane proportional to the intensity of the incident IR radiation, and this temperature rise is then measured using a thermal detector or other electro-thermometer.
The amount of temperature rise and thus the detector sensitivity is inversely proportional to the thermal conductance of the supports by which the absorber is suspended. Thus, the thermal conductance of these supports limits the sensitivity to less than the theoretical background limit that could be attained. In addition, attempts to reduce the conductance come at the expense, however, of the response time of the detector. That is, when the thermal conductance is rendered relatively low, the membrane temperature would then come into equilibrium slowly with its substrate and thereby limit its response time to new measurements.
U.S. Pat. No. 5,523,564 discloses an infrared detecting element that has a supporting member for supporting an infrared detecting member above a substrate with a low thermal conduction part intervening between the substrate and the support member. The support member can be actually fabricated by etching a portion of the substrate to provide supporting posts over a cavity portion with the supporting posts being porous with low thermal conductivity. The resulting structure is normally operated in a vacuum and since there is neither a solid material nor air to conduct heat between the membrane and the support, the only path of thermal conduction between them is through the relatively thin or porous supports so that the thermal conductance between the membrane and the heat sink substrate can be maintained relatively low.
In operation, the thermal radiation can be absorbed on the membrane, either directly on the membrane or by a specially added absorbing material. This causes the temperature of the membrane to rise, and this temperature is then measured by some sort of electrothermometer. The difference between the substrate and the membrane temperature is proportional to the intensity of the incident infrared radiation and can thereby be converted into electrical signals as the detector's output signal. The incident IR radiation constitutes a flux of heat into the membrane, and the heat flow out of the membrane is equal to the temperature difference between the membrane and the substrate multiplied by the thermal conductance of the supports. This temperature difference will increase until the heat flow out of the membrane is equal to the heat flux into the membrane. The membrane temperature will rise and fall with a time constant equal to the heat capacity of the membrane divided by the thermal conductance of the supports.
Due to system noise and other limitations, there is a minimum temperature difference that a thermometer can accurately detect. The amount of incident infrared radiation necessary to cause this minimum detectable temperature change is proportional to the thermal conductance of the supports by which the membrane is suspended. Therefore, the sensitivity, that is the minimum detectable amount of incident radiation, depends upon the thermal conductance of the supports. Lowering the thermal conductance of the supports, if possible, would come at the expense of the response time of the infrared detector. That is, the membrane would then come to equilibrium more slowly, if the thermal conductivity is reduced.
Another example of an infrared detector can be found in U.S. Pat. No. 5,640,013, which discloses a bolometer-type of infrared sensor with a high concentration impurity layer formed to enhance its detection sensitivity.
U.S. Pat. No. 5,602,393 discloses a micro-bolometer detector with enhanced sensitivity provided by the incorporation of an optically absorptive material structure tuned so that a predetermined design wavelength can be redirected back to cause constructive interference to thereby increase the absorption of optical radiation at the design wavelength.
U.S. Pat. No. 5,597,957 is cited for its disclosure of a microvacuum sensor providing a heating element, such as aluminum, arranged on a thin membrane. The microvacuum state is determined as a result of the thermal conduction of the gas that surrounds the sensor chip which is in turn influenced by the gas pressure to thereby enable the determination of the vacuum or gas pressure that exist.
U.S. Pat. No. 5,602,389 discloses a calibration of an infrared sensor with a black body.
U.S. Pat. No. 4,694,175 discloses a mounting arrangement of an infrared detector wherein a cold finger is used to reduce temperature variations in the detector.
U.S. Pat. No. 5,683,181, U.S. Pat. No. 5,602,043, and U.S. Pat. No. 5,572,312 are cited of general interest.
The prior art is still seeking to improve both the sensitivity and response time of a thermal infrared detector so that it can come closer to approximating its theoretical potential.